4 Ways to Avoid the Bike-Run Bonk

In 2006, I flew to Penticton, British Columbia, to watch a couple of the athletes I coach compete in Ironman Canada. One of them had a day to remember; the other had a day to forget.

The latter, Paul, was doing fine through the end of the bike leg. He arrived at T2 right on his goal pace. But things fell apart quickly on the run. Almost immediately his stomach began to feel bloated and sloshy. His legs grew heavy and seemed starved for energy, and soon he was even experiencing some lightheadedness. By the 5K mark of the marathon he was walking.

This scenario—which I call the bike-run bonk—is common in long-distance triathlons. The athlete feels good or at least OK on the bike, only to suffer a gastrointestinal meltdown early in the run. Fortunately, as common as the bike-run bonk is, it is completely avoidable.

Knowing is Half the Battle

The key to avoiding the bike-run bonk is understanding exactly what it is. The bike-run bonk is a simple case of over-nourishment with a twist. The twist is that the stomach is able to tolerate a greater volume and concentration of nutrition, and is also able to empty more quickly, when an athlete is bicycling than when that same athlete is running. So what qualifies as optimal nourishment during the bike leg of a triathlon suddenly becomes over-nourishment on the run.

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The essential difference between cycling and running with respect to nutrition is the far greater amount of stomach jostling that occurs on the run. This jostling is the likely cause of the unpleasant sloshy feeling that often becomes full-blown nausea if the stomach volume is too great.

Stomach jostling probably also contributes to a reduced gastric emptying rate (i.e. slower absorption of nutrition through the stomach and intestine) during running as compared to cycling. The result is a nutrition backlog in the stomach, small intestine and possibly the colon that's not unlike the damming of a river and subsequent flooding of riverfront properties.

Such a backlog and the resulting accumulation of fluid in places it should not be (e.g. the colon) is also a cause of that terrible bloated feeling.

If that wasn't bad enough, when your pipes get stopped up in this manner a secondary problem results: inadequate supply of fluid and energy to your blood and muscles, which can quickly result in a classic energy bonk. Isn't that ironic?

You crammed all that nutrition down your throat on the bike to prevent dehydration and glycogen depletion and it winds up causing these very things—in addition to gastrointestinal distress.

A Few Ounces of Prevention

A key cause of the bike-run bonk, then, is taking in too much nutrition (and perhaps too high a concentration of nutrition) during the latter portion of the bike leg. It's not too much with respect to the latter portion of the bike leg itself, but it becomes too much in the early portion of the run leg.

The way to avoid the bike-run bonk is to fuel yourself during the final 30 minutes of the bike leg in a way that anticipates the reduced capacities of your stomach on the run. Here are four specific tips to help you avoid the bike-run bonk.

1. Go Light

Throughout the majority of the bike leg, take full advantage of the opportunity to take in fluid and energy at a high rate. A typical cyclist can absorb 1.2 to 1.5 liters of fluid and 80 to 100 grams of carbohydrate per hour at race intensity. You can also tolerate a fairly full stomach on the bike, and it's a good idea to keep your stomach as full as you comfortably can by taking in nutrition frequently, because the fuller your stomach is, the faster it empties.

But with around 30 minutes remaining in the bike leg you must sharply reduce your rate of nutrition intake and allow your stomach volume to come down to a level that is manageable for the run. I recommend taking an energy gel with water or a few swigs of a sports drink with 30 minutes to go and another drink with 15 minutes to go, and that's all. If it's hot, drink at 30 minutes, 20 minutes and 10 minutes.

This advice is precisely the opposite of what I hear many coaches and triathletes preaching. They encourage long-distance triathletes to stock up on nutrition toward the end of the bike leg for the same reason I'm telling you to cut back—because it's impossible to consume nutrition at as high a rate on the run.

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